Traditionally dyes have been shipped to users in the textile, paper and soap industries in the form of dyestuff slurries; as wet cake from the final filtration step; or if shipped dry, in fiber drums.
In the wet form i.e. as slurries or wet cake, the dyestuffs must be formulated for rapid dispersability and stability of the liquid or semisolid state during storage and shipping. Often the stabilizing agents added, adversely affect the substantivity of the dyes for their intended use. In addition should the slurries or wet cake dry out, the resultant powders are difficult to re-wet as the powdered dyes tend to float on the water and require special surfactants and treatment.
The surfactants used for re-wetting often interfere with the application of the dyestuffs to the final substrate as they may interfere with the charge on the dye molecules.
Attempts have been made to ship the dyes in dry form packed in fiber drums. If shipped in powder form as dried from the slurry or wet-cake, the dyes are usually extremely light and fine powders. Not only are such powders difficult to re-wet, as noted above, but the powders are all pervasive, coloring and contaminating the surroundings. Additionally, the extremely fine light powders, floating in the air as dusts (under 200 microns in diameter), can present explosion hazards.
The economic drawbacks of shipping slurries or wet cakes include the payment for shipping the large amounts of water, usually 25 to 60% water content. The fiber drums used for the dry powders have a weight drawback when the metal bottoms and closures are considered in the total container weight.
To overcome some of the above problems, the past practice, particularly in the fields of fluorescent whitening agents and basic dyes, which are shipped in tremendous quantities for textiles and soap use, has been to spray-dry the dyes.
Spray-drying of these dyes results in the formation of granules or spheres of the dye which remain after the water from the slurry is rapidly removed. By control of the dryer feed using spray nozzles with specific orifices, to define shape spray pattern and direction and particle size, or by use of a rotating disk atomizer, it has been possible to form granules or spheres which can be rapidly dried in the spray chamber, with minimum particle attrition. However, the resulting granules have little compression strength due to the requirement for rapid moisture removal and inherent lack of particle compression during spray granule formation. Attempts had been made to strengthen the inherent integrity of the granules by adding binders, such as gums or sodium silicate to the slurry before spraying, but such additives were often undesirable and did not afford sufficient strength.
Because of the lack of strength of the spray-dried dye spheres, it has been necessary to ship them in fiber drums. When stacked-bag, shipping was attempted it was found that the weight of stacked bags reduced the spheres in the bags at the bottom layer to unacceptable powder. As mentioned above, such powders are difficult to rewet, contaminate the surroundings with their pervasive color or fluorescence and can present an explosive hazard.
Economically the most important of such dyes are the fluorescent whitening agents (FWA).
These are used in textile manufacture where they are added to the polymers during fiber formation and during dyeing after spinning, weaving or knitting and during thermosetting resin treatments such as for wash and wear. Even more of these dyes are used by "soapers", the manufacturers of laundering detergents and aids. Particularly useful and used in tens of million of pounds annually as brighteners, are the triazinyl stilbene fluorescent whitening agents (FWA's), marketed under the Tinopal.RTM. trademarks, by Ciba-Geigy.
It is an object of this invention to provide a method for producing dyes and particularly forms of FWA, which are in granular form, directly from filter cakes or slurries.
More particularly, it is an object of this invention to provide the dyes in granular form of sufficient strength to withstand the rigors of shipment in bags without the need for binders. In addition, as an ancillary object, the explosion hazard from powdered dyes should be reduced.
A further object is the provision of a method for preparing dye granules which are standardized for assay (tintorial, optical density, or fluorescence) during their preparation and which require no further standardization after drying.
It is also an important object that the dry granules from the process of this invention be readily dispersable and soluble in aqueous systems without the addition or recourse to wetting agents or dispersing agents.